The present invention relates generally to pipe snubbers and more particularly to a pipe snubbing system for simultaneously locking or unlocking a set of pipe snubbers based on the output of one or more strategically positioned sensors.
The function of pipe snubbers is to provide lateral restraint for process piping during a seismic or other abnormal vibration event, while permitting normal pipe displacements at other times, such as normal vibrations, expansion and contraction, and other acceptable pipe motion. To perform this function, the snubber "locks up" (i.e., effectively becomes a rigid strut, tying the process pipe to the facility wall). The purpose for providing lateral restraint for the pipe during a seismic event is to minimize the possibility of pipe "whip" (large amplitude pipe swaying or vibration) which in turn would result in high pipe stresses, possibly leading to failure of the pipe with inherent serious consequences to the facility. Ideally, the snubbers would not, at all other times, apply any side load to the pipe.
Snubbers are used to protect process piping for any applications where pipe failure, as a result of a seismic event, would have serious safety or other consequences. Applications at the U.S. Department of Energy's Hanford facility include the primary and secondary coolant system piping of the FFTF (Fast Flux Test Facility) and the N Reactor. The FFTF uses about 3,300 mechanical snubbers while the N Reactor employs about 1,000 hydraulic snubbers. Snubbers are used extensively in chemical plants, pipeline pumping stations and in fossil and solar power plants, as well as in nuclear power plants. The same snubbers may be utilized in nuclear and non-nuclear applications as long as the lubricants and materials used in constructing the snubbers are not significantly effected by radiation.
Existing snubbers include the hydraulic type and the mechanical type. One existing hydraulic snubber uses a control valve, with no moving parts, which is built into the piston. The valve allows fluid flow through the piston and hence piston rod movement under normal conditions (i.e., when the pipe is moving slowly relative to the building, as a result of temperature changes), but will react to block fluid flow and hence piston rod movement under strong pipe force conditions (i.e., large and/or rapid pipe displacements). One existing mechanical snubber has a lead screw/ball nut arrangement rotating a flywheel. Only strong pipe forces (i.e., rapid, large pipe displacements) will rotate the flywheel fast enough so that a centrifugal-force-responsive brake is actuated to lock the ball nut with respect to the lead screw. About half of the commercial power producing reactors in this country use hydraulic snubbers; the other half use mechanical snubbers. In all cases, the snubbers employed are isolated units: each must individually sense the increase in g loading (sensing either pipe displacement or acceleration) resulting from a seismic event on the associated process pipe, and each must react by "locking up" at a predetermined g loading, unlock when the g loading is reversed, and again "lock up" to limit displacement in the opposite direction. This cycle is repeated until the amplitude of the pipe displacement or acceleration is reduced below the level at which the device is triggered to "lock up".
A review of the current thinking regarding alternate snubber design concepts indicates that the effort is on finding other, hopefully more reliable, mechanical devices, for restraining rapid pipe motion. For example, in New Zealand, an effort is being made to adapt clamped flat plates sliding relative to each other to perform this function. In this country, a concept utilizing a hoop interposed between the pipe and the facility wall has been proposed to allow slow displacments of the process pipe while restraining large, rapid transient motions.
As noted above, the state-of-the-art snubbers depend upon each individual snubber sensing the relatively rapid motion of the attached pipe which occurs during a seismic event. Thus each snubber acts as an individual unit. During a seismic event, each will be activated at g loadings (accelerations and velocities) which depend upon the individual snubber's set point as determined by its original manufacture and modified by its current status regarding wear and maintenance. Thus, at any time during a seismic event, some of the snubbers will be "locked-up" (actuated). Others will still allow free motion of the pipe. Note also that each type of state-of-the-art snubbers, by its design, applies a continual side load of undetermined magnitude on the adjacent pipe. There is no functional benefit resulting from this drag. It is, however, inherent in all state-of-the-art snubbers.
Hydraulic snubbers generally fail as the result of seal leakage. This is usually first evidenced by hydraulic fluid on the floor below the snubber. This external leakage is thus easily noted. Each snubber has a small reservoir, and the operability of the individual snubber requires that the cylinder and tubing be full of hydraulic fluid (and devoid of air). Thus, the length of time a snubber is serviceable, as far as external leakage is concerned, depends on the external leak rate and quantity of reserve fluid. When the reservoir empties, air can enter the snubber's cylinder, tubing and valve arrangements. When this occurs, the snubber will no longer restrict the pipe motion as required. That is, it will not "snub". Thus, because of leaking external seals, the hydraulic snubbers currently in use require considerable personnel radiation exposure and reactor shutdown time for maintentance.
Another problem is related to the fact that even if the external seals do not leak, there is no way to determine, in situ, if each of the snubbers will perform its function when required or if it is presently and undesirably "locked up".
Mechanical snubbers depend upon inertia to operate a brake to cause the snubber to "lock up". During normal heat up and cool down of the process piping, the rate of motion is relatively small. The snubber applies a small side force on the pipe. This force slowly rotates the flywheel. When a seismic event occurs, the flywheel rotational velocity increases to the point at which centrifugal force causes the brake to actuate, momentarily stopping the flywheel and thus the longitudinal motion. Upon reversal of direction of the pipe motion, the brake on the flywheel releases, the flywheel begins rotating rapidly in the opposite direction, and the centrifugal brake again operates. It is obvious that wear, maintenance work, repeated tests, and adjustments will have an effect on reliability and upon the set point at which "lock up" occurs. To determine the condition of the individual snubbers, i.e., whether each is "locked up" or is free and ready to perform its function, it is necessary to remove it from its mountings and take it to a shop or return it to the manufacturer for test and recertification of its "lock-up" setting. Thus, inspection, test, and maintenance over the life of the facility will be costly in terms of maintenance craft hours, personnel radiation exposure, and outage time required to disassemble the snubbers from their installed positions and to check out and maintain the individual snubbers. Repair of snubbers will be expensive since "clean room" repair conditions are recommended by the manufacturer. Degradation of lubrication contributes further to maintenance costs and to changes in the set point at which "lock up" occurs.
Since the mechanical snubbers available commercially utilize a ball nut-lead screw and flywheel, significant normal vibration of the pipe can cause excessive wear of the "continually" reciprocating parts. At particular amplitudes of vibration and at particular frequencies, excessive wear and premature "lock up" can occur. It should be noted that premature "lock up" can cause excessive pipe stresses by restraining the motion of the pipe during thermal transients.